The present invention relates to the general field of reducing noise from the jet leaving a turbomachine nozzle. It relates more particularly to a cover for the nozzle of a separate-stream type turbomachine, which cover is provided with patterns for reducing noise from the jet.
Nowadays sound pollution has become a major concern for engine manufacturers who are being challenged more and more concerning the sound nuisance of their turbomachines. The sources of noise in a turbomachine are numerous, but it has been found that the noise from the jet leaving the nozzle is the predominant noise during aircraft takeoff. Since certification authorities are becoming more and more difficult to satisfy in terms of sound emission from turbomachines, engine manufacturers have been required to make efforts to reduce noise from their turbomachines, and in particular noise from the jet leaving the nozzle.
Typically, a separate-stream nozzle of a turbomachine comprises a primary cover coaxial about the longitudinal axis of the turbomachine, a secondary cover disposed coaxially around the primary cover so as to define a first annular channel along which an outer stream (or cool stream) flows, and a central body disposed coaxially inside the primary cover so as to define a second annular channel along which an inner stream (or hot stream) flows, the primary cover extending beyond the secondary cover.
In such a nozzle, the jet noise comes from mixing between the cool and hot streams and between the cool stream and the outside air flowing round the nozzle. This noise is noise over a broad frequency band generated by two types of sound source: high frequency noise coming from small turbulent structures in the mixing between the cool and hot streams and audible essentially in the immediate vicinity of the nozzle; and low frequency noise coming from large turbulent structures that appear at a distance from the jet.
In order to reduce jet noise, one of the means used is to increase the effectiveness of the mixing between the streams. For this purpose, it is well known to provide one of the covers of the nozzle with a plurality of repeated patterns distributed around the circumference of the trailing edge of the cover. By installing such patterns at the trailing edge of the nozzle cover, mixing between the streams is achieved by creating vortices close to the nozzle so as to better dissipate kinetic energy, and consequently reduce the turbulent intensity of large vortices constituting the major sources of noise.
By way of example, U.S. Pat. No. 6,532,729 describes providing the trailing edges of the primary and secondary covers of the nozzle with a plurality of repeated patterns of triangular shape (referred to as “chevrons”) that serve to encourage mixing between the hot and cool streams. Similarly, patent publication US 2002/0164249-A1 proposes providing the trailing edges of the primary and secondary covers of the nozzle with a plurality of repeated patterns of trapezoidal shape (referred to as “crenellations”).
Although encouraging mixing between the streams, the patterns mentioned above are not entirely satisfactory. Even if such patterns enable the low frequency component of the jet noise to be reduced, that is generally achieved to the detriment of its high frequency component which remains at a level that is too high.